1. Field of the Invention
The present invention relates to a method and an apparatus for measuring a defocus value and an exposure light amount necessary for obtaining satisfactory imaging performance in a projection exposure apparatus that is used to manufacture, e.g., a semiconductor element, a liquid crystal display element, or a thin-film magnetic head by lithography. The present invention also relates to a method of controlling a measured defocus value and an exposure light amount. The present invention further relates to an exposure apparatus and a device manufacturing method capable of measuring and controlling a defocus value and an exposure light amount.
2. Description of the Related Art
To manufacture, e.g., a semiconductor element, a liquid crystal display element, or a thin-film magnetic head by lithography, a projection exposure apparatus is used, which forms a pattern image of a mask or a reticle (to be referred to as a “reticle” hereafter) on a photosensitive substrate through a projection optical system.
In recent years, the degree of integration of semiconductor elements has risen, and the process line width has decreased. To cope with this, a projection exposure apparatus uses a projection lens with a higher NA and a light source with a shorter wavelength and a larger angle of view. As a unit configured to achieve these, an apparatus called a stepper is used. A stepper reduces an almost square exposure area and exposes it to a wafer by cell projection. Another up and coming mainstream device is a scanning exposure apparatus called a scanner, which forms a rectangular slit-shaped exposure area and scans a reticle and a wafer relative to each other at a high speed, thereby accurately exposing a wide area. Under these circumstances, managing the defocus value and exposure light amount of the projection exposure apparatus is increasingly becoming important to maintain the allowable line width accuracy of a pattern as the numerical aperture (NA) of the projection lens rises, and the exposure wavelength shortens.
The following three methods are conventionally used to measure a defocus value.
(1) Registration marks are transferred and exposed to a wafer by using a special reticle, and the positions of the exposed registration marks are measured (Japanese Patent Laid-Open Nos. 2002-55435 and 2002-289494).
(2) A line length is measured by using LES (Line End Shortening) (Japanese Patent Laid-Open No. 1-187817 and U.S. Pat. No. 5,965,309).
(3) The sidewall angle information of a resist is measured using an SEM, an optical CD measuring device or an AFM (U.S. Pat. No. 6,150,664 and Japanese Patent Laid-Open No. 2003-142397).
The methods (2) and (3) can measure not only a defocus value, but also, an exposure light amount using the same mark.
However, these conventional techniques have the following problems.
In method (1), the light that has passed through the registration mark must form an asymmetrical incidence angle distribution on the wafer. For example, in Japanese Patent Laid-Open No. 2002-55435, registration marks on the reticle have a phase difference of 90°. In Japanese Patent Laid-Open No. 2002-289494, a special reticle prepared by, e.g., forming patterns on both surfaces is used. This makes it difficult to form both an element pattern and registration marks on a reticle. Hence, this method is not suitable for measuring the focus and exposure light amounts of an element pattern in a mass production line. The method is usable only for, e.g., periodic maintenance of the curvature of field of a projection exposure apparatus.
Method (2) requires no special reticle and allows measurement using a relatively inexpensive measuring device. However, the line length of the measurement target exhibits a parabolic behavior on both sides of the best focus position. For this reason, although it is possible to measure the absolute value of the defocus value, the sign of the value is unknown. To determine the sign of focus, it is necessary to change the focus by a predetermined amount, to execute exposure and light length measurement again, and then, to determine the sign on the basis of the increase or the decrease in the line length.
Method (3) requires an expensive measuring device, such as an optical CD (Critical Dimension) measuring device or an AFM to execute measurement in the CD direction, unlike the methods (1) and (2), which execute measurement using an optically sensed image. This poses a problem in the measurement accuracy as the pattern becomes small.
A prior art arrangement and its associated problem will be described in detail with reference to FIGS. 18A to 18C and 19. FIGS. 18A to 18C are graphs, each showing the relationship between the defocus value and the pattern length. The abscissa represents the defocus value, and the ordinate represents the pattern length. FIG. 19 is a view for explaining the focus amount or the exposure light amount measurement method disclosed in Japanese Patent Laid-Open No. 1-187817. As shown in FIG. 19, wedge-shaped registration marks RP are transferred and exposed to a wafer. Then, a slit beam SP scans in the direction of the arrow. A pattern length Ly is obtained from the diffracted light intensity. When the focus position of the exposure apparatus changes, the pattern length Ly, which has a characteristic such as a quadratic function, reaches the maximum at the best focus position and decreases as the light defocuses, as shown in FIG. 18A. Such a focus characteristic curve is acquired in advance, and the pattern length Ly of the registration mark RP on the exposed wafer, as the inspection target, is measured. When only the pattern length Ly=Ly1 is measured, two values F1 and F2 are available as the focus inspection values, as shown in FIG. 18B. It is, therefore, impossible to determine the inspection value corresponding to the pattern length. Conventionally, as shown in FIG. 18C, after the focus position of the exposure apparatus is moved by dF from that in the first exposure, the registration marks are exposed, and the pattern length Ly is measured again. If the pattern length Ly of the second time is Ly2, the focus inspection value of the first time is determined to be F1. If the pattern length Ly is Ly2′, the focus inspection value of the first time is determined to be F2. That is, in the conventional inspection method using Line End Shortening, only one exposure process is insufficient to determine the sign of focus because of the quadratic function-like characteristic with the maximal value at the best focus position. Additionally, since the pattern length change characteristic has a quadratic function with the maximal value at the best focus position, and the change in the pattern length near the best focus position is small, the resolution in focus measurement lowers.
As described above, there is no measuring method or apparatus having a sufficient function as an inline focus monitor (or inline exposure light amount monitor), which inspects the focus amount and exposure light amount and appropriately corrects them in accordance with a secular change, in a semiconductor manufacturing mass production line.